1. Field of the Invention
This invention relates to an improvement in a vehicle equipped with an automatic transmission, and more particularly to a control system for an engine of a vehicle equipped with an automatic transmission which engine is so arranged that its idling engine speed is normally maintained slightly high.
2. Description of the Prior Art
In a vehicle equipped with an automatic transmission, an engine is mechanically connected through a torque converter with a transmission mechanism or planetary gear system. More specifically, the engine is connected to the pump impeller of the torque converter whose turbine runner is, in turn, connected to the transmission mechanism through a power input shaft and a forward clutch. Accordingly, when the automatic transmission is in a non-drive range in which the selector lever is in N (Neutral) or P (Park) position to release the forward clutch, the turbine runner of the torque converter rotates without connection to the transmission mechanism, together with the pump impeller driven by the engine. When the transmission is in a drive range in which the selector lever is in D (Drive), D2 (Second drive), D1 (First drive), or R (Reverse) position so as to disengage the forward clutch, the turbine runner is connected to the planetary gear system.
With the thus arranged automatic transmission, when the vehicle is at standstill and the transmission is in the drive range, i.e., the engine is operated at idling upon the transmission being in the drive range, the turbine runner of the torque converter is halting together with the planetary gear system. Then, the pump impeller stirs fluid within a converter chamber of the torque converter and consequently a considerable load is applied to the engine, thereby causing engine speed to lower so as to increase engine vibration while resulting in a possibility that the engine will halt. In view of the above drawbacks, hitherto in a vehicle equipped with an automatic transmission, the valve opening degree of the throttle valve at its fully closed position (at idling) is so set that the engine speed is slightly higher (usually, about 100 r.p.m.) than that in a vehicle equipped with a manual transmission.
However, the following problems have been encountered with the vehicle in which idling engine speed is uniformly increased as stated above: Even during coasting of the vehicle, i.e., vehicle cruising due to the force of inertia in which the automatic transmission is in the drive range and an acceleration pedal is not operated, the above-mentioned valve opening degree of the throttle valve at its fully closed position is maintained to increase the engine speed. As a result, in such coasting of the vehicle, engine speed is increased to consume unnecessary fuel while deteriorating the effect of engine braking.
Additionally, during idling of the engine in which the transmission is in the non-drive range, the turbine runner rotates without connection to the transmission mechanism and therefore idling engine speed is increased, thereby increasing fuel consumption while increasing engine noise.